Many of bacteria swim by rotating helical filaments that act as propellers. This motility is a factor in the virulence of many bacterial pathogens, including those that cause ulcers, syphilis, burn wound infections, and some diarrhea. Each filament is driven by rotary motor in the cell membrane; the filament/motor structure is called a flagellum. The energy for rotation comes from the membrane iron gradient. Like any rotary motor, the bacterial flagellar motor possesses a stator (the non-rotating part) and a rotor (the rotating part). FliG, FliN, and FliM are three proteins that function in a complex on the rotor. Recently, x-ray crystallography has been used to determine the three-dimensional structure of a domain of the rotor protein FliG. This domain functions directly in motor rotation, and is known to interact with proteins of the stator. This is the first high-resolution structure determined for any component of the flagellum. In the work proposed here, the FliG domain structure will be exploited to guide detailed biochemical and functional studies of this key rotor component. Another flagellar rotor protein, FliN, has also been crystallized and preliminary data show that it will be feasible to determine its structure. The structure of FliN will be determined, and also used to guide biochemical and functional studies. The long-term goal of this work is to understand the structure of the protein complex that forms the flagellar rotor, and to understand the mechanism of motor rotation in light of this structure. The proposed work will bring us significantly nearer this goal, by revealing structures and spatial relationship of rotor components in unprecedented detail.